Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits (ICs), and the like. A frequently used substrate for such applications is a semiconductor wafer. One skilled in the relevant art will recognize that the description herein also applies to other types of substrates. In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (e.g., a silicon wafer) that has been coated with a layer of radiation-sensitive material (e.g., resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (with M<1) the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference in its entirety.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (e.g., resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are desired, then the whole procedure, or a variant thereof, should be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference in its entirety.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens.” However, this term should be broadly interpreted as encompassing various types of projection systems, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. The position of a second element traversed by the projection beam relative to a first element traversed by the projection beam will for simplicity hereinafter be referred to as “downstream” of or “upstream” of the first element. In this context, the expression “downstream” indicates that a displacement from the first element to the second element is a displacement along the direction of propagation of the projection beam. Similarly, “upstream” indicates that a displacement from the first element to the second element is a displacement opposite to the direction of propagation of the projection beam. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices, the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and International Patent Application Publication No. WO 98/40791, incorporated herein by reference in their entireties.
There is a desire to integrate an ever-increasing number of electronic components in an IC. To realize this, it is desirable to decrease the size of the components and therefore to increase the resolution of the projection system, so that increasingly smaller details, or line widths, can be projected on a target portion of the substrate. Therein the wavelength of the radiation may play an essential role. The shorter the wavelength, the more transistors can be etched onto the silicon wafer. A silicon wafer with many transistors may result in a more powerful, faster and/or less power consuming microprocessor. In order to enable processing with light of a shorter wave length, chip manufacturers developed a lithography process known as Extreme Ultraviolet Lithography (EUVL). In this process, transparent lenses are replaced by mirrors that are arranged in a vacuum environment.
In order to actually realize that the details at such high resolution are imaged with sufficient accuracy, the projection system and the mirrors forming the lens elements used in the projection system should comply with very stringent quality requirements. Despite the great care taken during the manufacturing of lens elements and the projection system, they both may still suffer from wavefront aberrations, such as, for example, displacement, defocus, astigmatism, coma and spherical aberration across an image field projected with the projection system onto a target portion of the substrate. The aberrations are sources of variations of the imaged line widths occurring across the image field. The imaged line widths at different points within the image field should be constant. If the line width variation is large, the substrate on which the image field is projected may be rejected during a quality inspection of the substrate. Using techniques such as phase-shifting (e.g., using phase-shifting masks), or off-axis illumination, the influence of wavefront aberrations on the imaged line widths may further increase.
During manufacture of a lens element, it may be advantageous to measure the wavefront aberrations of the lens element and to use the measured results to tune the aberrations in this element or even to reject this element if the quality is not sufficient. When lens elements are put together to form the projection system, it may again be desirable to measure the wavefront aberrations of the projection system. These measurements may be used to adjust the position of certain lens elements in the projection system in order to minimize wavefront aberrations of the projection system.
After the projection system has been built into a lithographic projection apparatus, the wavefront aberrations may be measured again. Moreover, since wavefront aberrations are variable in time in a projection system, for instance, due to deterioration of the lens material or lens heating effects from local heating of the lens material, it may be desirable to measure the aberrations at certain instants in time during operation of the apparatus and to adjust certain movable lens elements accordingly to minimize wavefront aberrations. It may be desirable to measure the wavefront aberrations frequently due to the short time scale on which lens-heating effects may occur.
United States Patent Application Publication No. 2002/0145717, which is incorporated herein by reference in its entirety, describes a wavefront measurement method that uses within the lithographic apparatus a grating, a pinhole and a detector, e.g. CCD detector. The detector may have a detector surface substantially coincident with a detection plane that is located downstream of the pinhole at a location where a spatial distribution of the electric field amplitude of the projection beam is substantially a Fourier Transformation of a spatial distribution of the electric field amplitude of the projection beam in the pinhole plane. With this measurement system built into the lithographic projection apparatus it is possible to measure in situ the wavefront aberration of the projection system.
In another measurement, a transmission image sensor (TIS) is used as the detector to determine relative positions of the wafer and reticle stages. During a TIS scan, the wafer stage carrying the TIS modules, moves in 3D across the aerial images of the TIS object marks on the reticle (or fiducial) created by the projection optics of the scanner. In the case of EUV lithography such aerial images for the TIS fine scans are narrow, e.g. in the order of 50 nm wide lines at wafer level. The sensor marks on the TIS carry similar lines (100 nm wide). A perfect overlap of the object and the corresponding sensor mark results in a maximum signal on the detector. A scan with a combination of x- and y-marks (lines along y and x directions) gives the aligned position, i.e., when the position of the wafer stage (x,y,z) at which the TIS-sensor mark is aligned to an aerial image of a TIS-object mark at a given reticle stage position and orientation (x, y, z, Rx, Ry, Rz).
The signals generated by the detector are relatively weak. Hence it is important that the signals from the detector are pre-processed by an electronic circuit that is arranged close to the detector, in order to prevent that the signals to be processed are corrupted by noise. In an EUV lithographic apparatus, the detector module is however arranged in a hostile environment. The radiation impinging on the detector module causes a strong heat development. As indicated above, the environment should be vacuum to prevent absorption of the EUV radiation. Said vacuum environment in which the sensor is arranged does not allow for heat transport by convection or conduction. Furthermore, EUV radiation may form a source for electrostatic discharge as the EUV radiation results in photo-electron generation when it is absorbed by a surface of the detector module.
A further complication is that only a limited amount of space is available.
In view of the above, there is a need for a sensor arrangement that is capable of performing accurate optical measurements despite these hostile operational circumstances.